1. Field of the Invention
The present invention relates to a junction structure of dielectric strips which are built in a millimeter-wave integrated circuit and the like to transmit, branch and synthesize high-frequency signals, a nonradiative dielectric waveguide using the junction structure, and a millimeter-wave transmitting/receiving apparatus.
2. Description of the Related Art
A nonradiative dielectric waveguide (hereinafter referred to as an NRD guide) S1 using a conventional dielectric strip for transmitting high-frequency signals of tens GHz is shown in FIG. 17. FIG. 17 is a partially cutaway perspective view of the NRD guide S1, which is formed by joining, above and below a dielectric strip 2 having a rectangular section, parallel plate conductors 1, 3 each having a major surface larger than the top and bottom surfaces of the dielectric strip 2. In the NRD guide S1, in the case where the spacing between the parallel plate conductors 1, 3 is equal to or less than xcex/2 (xcex denotes a wavelength of high-frequency signals), high-frequency signals with a wavelength more than xcex are cut off and incapable of entering the spacing between the parallel plate conductors 1, 3. The dielectric strip 2 is interposed between the parallel plate conductors 1, 3, whereby high-frequency signals can propagate inside and along the dielectric strip 2, and radiation waves from high-frequency signals are suppressed by a cut-off effect of the parallel plate conductors 1, 3. The value xcex is equal to a wavelength of high-frequency (electromagnetic wave) signals propagating in the air. In addition, FIG. 17 is illustrated by cutting away part of the upper parallel plate conductor 3 in order to make the inside visible.
In order to branch high-frequency signals at a midway point of a dielectric strip in such an NRD guide, as shown in FIG. 18, a technique of mounting dielectric strips 11, 12 for branching high-frequency signals in the vicinity of a terminal of a dielectric strip 10 in which high-frequency signals are entered and propagated, and further mounting dielectric strips 13, 14 for propagating high-frequency signals in the vicinity of terminals of the dielectric strips 11, 12, respectively, has been put forth (refer to Papers of the Institute of Electronics, Information and Communication Engineers, C-I Vol.J75-C-I No.1, pp.35-41, January 1992). In this case, the dielectric strip 10 and the dielectric strips 11, 12, and the dielectric strips 11, 12 and the dielectric strips 13, 14 are placed at predetermined spacings so that high-frequency signals are spatially electromagnetically coupled. Besides, at the terminal of the dielectric strip 10 and the tips of the dielectric strips 13, 14, mode suppressors 15 for eliminating unnecessary transmission modes are placed. FIG. 18 is illustrated in perspective of the inside.
Further, as another construction of branching high-frequency signals at a midway point of a dielectric strip in an NRD guide, as shown in FIG. 19, a technique of installing a straight dielectric strip 20 and a curved (U-shaped) dielectric strip 21 so that a curved protrusion of the dielectric strip 21 is in proximity to a midway point of the dielectric strip 20 is well-known (see Japanese Unexamined Patent Publications JP-A 6-174824 (1994) and JP-A 8-8621 (1996), and IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. MTT-31, No.8, August 1983, pp.648-654). In this NRD guide S3, part of high-frequency signals entered from an input port 20a of the dielectric strip 20 are propagated in the dielectric strip 20 and outputted from an output port 20b, and the rest thereof are spatially electromagnetically coupled at the curved protrusion of the dielectric strip 21 and outputted from an output port 21c. The dielectric strip 21, which is called a coupler, has a nonreflective terminator 22 at an end thereof opposite to the output port 21c and suppresses reflection of high-frequency signals at the nonreflective terminator 22. Here, FIG. 19 is illustrated in perspective of the inside.
The spacing L between the two dielectric strips 20, 21 at the proximate portion thereof is regulated, whereby high-frequency signals can be distributed at a desired branching ratio. It has been general in an NRD guide to distribute high-frequency signals by using a coupler as shown in FIG. 19.
On the other hand, the NRD guide S2 as shown in FIG. 18, in order to match the electromagnetic coupling among the dielectric strips 10-14, needs to place the dielectric strips 10-14 by precisely regulating the spacing thereof, and the component count thereof is considerably high, so that the practical utility thereof is low.
Therefore, an NRD guide using a coupler as shown in FIG. 19 is dominant, whose transmission property of high-frequency signals by frequency is shown in FIG. 20. Regulation is made in a manner that, when high-frequency signals of 60 GHz are entered from the input port 20a, the high-frequency signals are divided into halves with almost the same levels and outputted from the output ports 20b, 21c. Sba denotes an output level of high-frequency signals exiting from the output port 20b, and Sca denotes an output level of high-frequency signals exiting from the output port 21c. As shown in FIG. 20, the output levels Sba, Sca are largely varied, respectively, when the frequency is shifted from 60 GHz. Therefore, the conventional NRD guide S3 can be used only within a bandwidth of about 1 GHz centered at 60 GHz, exhibiting an insufficient frequency response in the field of communication devices such as a cellular phone which need to be usable in a wide band.
Further, in the NRD guide S3, the output levels Sba, Sca are largely varied when the spacing L between the dielectric strips 20, 21 is varied in FIG. 19, and hence the dielectric strips need to be placed with high accuracy, so that mass productivity of the NRD guide S3 has been prevented from enhancing. In addition, the dielectric strip 21 need to have the nonreflective terminator 22 at one end thereof, and in the case where the NRD guide is used at 60 GHz, the nonreflective terminator 22 becomes approximately 4-20 mm long, whereby downsizing of the NRD guide S3 has been hindered, and design thereof has been restricted.
Therefore, the present invention, which was made in view of the circumstances mentioned above, is aimed at providing an NRD guide which can be used in a wider band than the conventional one and hence applicable to devices used in a wide band such as communication devices, does not require precise positioning of a dielectric strip and thereby enhances mass productivity thereof, and does not need a nonreflective terminator disposed to a dielectric strip and hence can be designed with high flexibility and downsized.
The invention provides a junction structure of dielectric strips comprising a first straight dielectric strip for propagating high-frequency signals and a second dielectric strip which is joined to the first dielectric strip at a midway point thereof, wherein a junction between the second dielectric strip and the first dielectric strip is formed along an arc and the radius of curvature thereof is equal to or more than the wavelength of the high-frequency signals.
With the construction mentioned above, the invention can be produced in a state where the first dielectric strip and the second dielectric strip are integrated, and does not require precise positioning as in the case of individually placing these dielectric strips, so that mass productivity thereof is enhanced. Moreover, the second dielectric strip does not need to have a nonreflective terminator, so that the invention is highly flexible in design and advantageous for downsizing. In addition, the radius of curvature of the junction of the second dielectric strip is set to be equal to or more than the wavelength of high-frequency signals, so that the invention can be used in a wide band in a state where output levels of distributed high-frequency signals are almost equal to each other, thereby finding wide application to communication devices such as a cellular phone.
Further, the invention provides a nonradiative dielectric waveguide comprising the junction structure of dielectric strips disposed between parallel plate conductors placed at a spacing of xcex/2 or less with respect to a wavelength xcex of high-frequency signals.
With such a construction, the nonradiative dielectric waveguide of the invention can suppress radiation components from the dielectric strips to propagate high-frequency signals with high efficiency, and can be used in a considerably wider band, so that a general versatility thereof to a communication device, millimeter-wave radar or the like containing a millimeter-wave integrated circuit is increased.
The nonradiative dielectric waveguide of the invention comprises a first straight dielectric strip and a second dielectric strip which is joined to the first dielectric strip at a midway point thereof, wherein a junction between the second dielectric strip and the first dielectric strip is formed along an arc and the radius of curvature thereof is equal to or more than the wavelength of the high-frequency signals. Therefore, the invention can be produced in a state where the first dielectric strip and the second dielectric strip are integrated, and does not require precise positioning, so that mass productivity thereof is enhanced. Moreover, the second dielectric strip does not need to have a nonreflective terminator, so that the invention is highly flexible in design and advantageous for downsizing. In addition, the invention can be used in a wide band in a state where output levels of distributed high-frequency signals are almost equal to each other, thereby increasing a general versatility to a high-frequency circuit and finding wide application to a communication device such as a cellular phone, millimeter-wave radar or the like.
In the nonradiative dielectric waveguide of the invention it is preferable that the radius of curvature of the junction between the second dielectric strip and the first dielectric strip is in a range of from xcex to 3xcex.
According to the invention, the radius of curvature of the junction between the second dielectric strip and the first dielectric strip is selected to be in a range of from xcex to 3 xcex, whereby the nonradiative dielectric waveguide is capable of distributing high-frequency signals at nearly equal output strengths and therefore has an advantage in downsizing.
In the nonradiative dielectric waveguide of the invention it is preferable that in the case where the second dielectric strip is elongated along an arc from the junction toward the first dielectric strip, the second dielectric strip is formed so that a tangent of the elongated portion thereof comes in contact with a side wall of the first dielectric strip.
According to the invention, the tangent of the second dielectric strip elongated from the arc-shaped junction comes in contact with a side wall of the first dielectric strip, whereby the nonradiative dielectric waveguide is capable of equally distributing high-frequency signals.
In the nonradiative dielectric waveguide of the invention it is preferable that a frequency of the high-frequency signals is equal to or more than 50 GHz.
In the case where the nonradiative dielectric waveguide of the invention constructed as described above is disposed to automotive millimeter-wave radar, millimeter-waves are guided through the first dielectric strip and applied to an obstruction around the automobile and other automobiles, and intermediate frequency signals are generated by synthesizing reflection waves with high-frequency signals guided through the second dielectric strip, and then analyzed, whereby the distance from the automobile to the obstacle and other automobiles, the moving speeds, the moving directions and the like can be determined.
In the nonradiative dielectric waveguide of the invention it is preferable that the parallel plate conductors are made of Cu, Al, Fe, Ag, Au, Pt or stainless steel.
According to the invention, the parallel plate conductors are made of Cu, Al, Fe, Ag, Au, Pt or stainless steel, whereby the nonradiative dielectric waveguide can obtain high electric conductivity and processibility.
In the nonradiative dielectric waveguide of the invention it is preferable that the first dielectric strip and the second dielectric strip are made of an organic resin material, an organic-inorganic composite or ceramics.
According to the invention, the first dielectric strip and the second dielectric strip are made of an organic resin material, an organic-inorganic composite or ceramics, whereby the nonradiative dielectric waveguide can be easily processed so as to be low-loss with respect to high-frequency signals, and mass-produced.
As shown in FIGS. 6-11, the invention provides a millimeter-wave transmitting/receiving apparatus comprising:
(a) a voltage-controlled oscillating portion 21 comprising:
a high-frequency diode 33 for outputting high-frequency signals of millimeter-wave band, and
a variable capacitance diode 30 placed so that a bias voltage applying direction 72 coincides with an electric field direction of the high-frequency signals, for outputting the high-frequency signals as frequency-modulated transmission millimeter-wave signals by periodically controlling bias voltage,
the voltage-controlled oscillating portion 21 being installed at an end of a first dielectric strip 37b (37a);
(b) a second dielectric strip 75 which is joined, along an arc having a radius of curvature r not less than the wavelength xcex of the transmission millimeter-wave signals, to a straight portion 37b1 of the first dielectric strip 37b on the downstream side from the voltage-controlled oscillating portion 21 in the direction 71 for transmitting the transmission millimeter-wave signals of the first dielectric strip 37b (37a);
(c) a circulator 76 which has an input end 78, an input/output end 79 and an output end 80,
the circulator 76 being connected to the other end of the first dielectric strip 37b at the input end 78,
for outputting transmission millimeter-wave signals inputted into the input end 78 to the input/output end 79, and
outputting reception signals inputted into the input/output end 79 to the output end 80;
(d) a third dielectric strip 77, one end of which is connected to the input/output end 79 of the circulator 76, and on the other end side of which is disposed a transmission/reception antenna 24;
(e) a fourth dielectric strip 81, one end of which is connected to the output end 80 of the circulator 76;
(f) a mixer 82 for connecting the second dielectric strip 75 and the fourth dielectric strip 81 to mix respective signals transmitted to the second and fourth dielectric strips 75, 81 to generate intermediate frequency signals; and
(g) a pair of conductor plates 84, 85 which are placed in parallel at a spacing equal to or less than one half of the wavelength xcex of the millimeter-wave signals, in which spacing are disposed the first to fourth dielectric strips 37a, 37b; 75, 77, 81, the voltage-controlled oscillating portion 21, the circulator 76 and the mixer 82.
The invention provides a millimeter-wave transmitting/receiving apparatus comprising:
(a) a high-frequency diode 33 which outputs high-frequency signals of millimeter-wave band;
(b) a first dielectric strip 37b (37a), one end of which is connected to the high-frequency diode 33, for propagating high-frequency signals outputted from the high-frequency diode 33;
(c) a pulse-modulating diode interposed between the first dielectric strip 37b (37a) or installed therealong, so that a bias voltage applying direction 72 coincides with an electric field direction of the high-frequency signals, for outputting transmission millimeter-wave signals which are pulse-modulated signals of the high-frequency signals by on-off of bias voltage;
(d) a second dielectric strip 75 which is joined, along an arc having a radius of curvature r not less than the wavelength xcex of the transmission millimeter-wave signals, to a straight portion 37b1 of the first dielectric strip 37b on the downstream side from the high-frequency diode of the first dielectric strip 37b (37a) in the transmission direction 71 of the transmission millimeter-wave signals;
(e) a circulator 76 which has an input end 78, an input/output end 79 and an output end 80,
the circulator 76 being connected to the other end of the first dielectric strip 37b at the input end 78,
outputting transmission millimeter-wave signals inputted into the input end 78 to the input/output end 79, and
outputting reception signals inputted into the input/output end 79 to the output end 80;
(f) a third dielectric strip 77, one end of which is connected to the input/output end 79 of the circulator 76, and on the other end side of which is disposed a transmission/reception antenna 24;
(g) a fourth dielectric strip 81, one end of which is connected to the output end 80 of the circulator 76;
(h) a mixer 82 for connecting the second dielectric strip 75 and the fourth dielectric strip 81 to mix respective signals transmitted to the second and fourth dielectric strips 75, 81 to generate intermediate frequency signals; and
(i) a pair of conductor plates 84, 85 which are placed in parallel at a spacing equal to or less than one half of the wavelength xcex of the millimeter-wave signals, in which spacing are disposed the first to fourth dielectric strips 37a, 37b; 75, 77, 81, the pulse modulating diode, the circulator 76 and the mixer 82.
In the millimeter-wave transmitting/receiving apparatus of the invention it is preferable that the portion 37b1 of the first dielectric strip 37b on the downstream side is curved so as to make an arc having the radius of curvature r and the second dielectric strip 75 is linearly connected to the arc-shaped portion.
As shown in FIG. 8, in the millimeter-wave transmitting/receiving apparatus of the invention it is preferable that the mixer 82 has a construction of electromagnetically coupling an arc-shaped portion 87 midway in a transmitting direction 86 of the second dielectric strip 75 to a straight or arc-shaped portion 89 midway in a transmitting direction 88 of the fourth dielectric strip 81, so as to be in close proximity to each other.
As shown in FIG. 9, in the millimeter-wave transmitting/receiving apparatus of the invention it is preferable that the mixer 82 has a construction of joining, to a straight portion 91 of the fourth dielectric strip 81, the second dielectric strip 75 along an arc-shaped portion 92 having the radius of the curvature r.
In the millimeter-wave transmitting/receiving apparatus of the invention it is preferable that the mixer 82 has a construction in which the second dielectric strip 75 is connected to the arc-shaped portion 91 of the fourth dielectric strip 81, having the radius of curvature r, so as to make a straight portion 92.
According to the invention, high-frequency signals of millimeter-wave band outputted by the high-frequency diode 33 are passed through the first dielectric strip 37a, a bias voltage of the variable capacitance diode 30 by a modulated wave which is periodically varied by a triangular wave or the like, transmission millimeter-wave signals from the voltage-controlled oscillating portion 21 composed of the high-frequency diode 33 and the variable-capacitance diode 30 are passed through the first dielectric strip 37b and outputted from the straight portion 37b1 of the first dielectric strip 37b through the input end 78 of the circulator 76 to the input/output end 79 of the circulator 76 to be radiated from a transmission/reception antenna 24 to a target 104. Reflection waves by the target 104 are supplied from the transmission/reception antenna 24 through the third dielectric strip 77 and guided from the input/output end 79 to the output end 80 of the circulator 76, and the fourth dielectric strip 81 and the second dielectric strip 75 of the mixer 82 are coupled, whereby intermediate frequency signals can be obtained. The mixer 82 may be constructed as shown in FIG. 8 mentioned later, or may be constructed as shown in FIG. 9.
It is possible that high-frequency signals of millimeter-wave band from the high-frequency diode 33 are pulse-modulated to be converted into transmission millimeter-wave signals. In this case, a pulse-modulating diode such as a pin diode or schottky-barrier diode is interposed midway in a transmitting direction 71 of the first dielectric strips 37a, 37b, or installed therealong, so that a bias voltage applying direction coincides with an electric field direction of the high-frequency signals, for converting the high-frequency signals into pulses by on-off of bias voltage. In the case where the pulse-modulating diode is interposed between the first dielectric strips 37a, 37b, as the pulse-modulating diode is used a pin diode having a constitution as shown in FIG. 11. In the case where the pulse-modulating diode is installed along the first dielectric strips 37a, 37b, another circulator is interposed between the first dielectric strips 37a, 37b, to an input/output end of which is connected another dielectric strip, at an end of which a schottky-barrier diode having a constitution as shown in FIG. 11 is provided. In this case, to input and output ends of the circulator are connected the first dielectric strips 37a, 37b. The millimeter-wave transmitting/receiving apparatus of the invention may comprise both the voltage-controlled oscillating portion 21 and the pulse modulating diode.
As shown in FIGS. 12-14, the invention provides a millimeter-wave transmitting/receiving apparatus comprising:
(a) a voltage-controlled oscillating portion 21 comprising:
a high-frequency diode 33 for outputting high-frequency signals of millimeter-wave band, and
a variable capacitance diode 30 placed so that a bias voltage applying direction 72 coincides with an electric field direction of the high-frequency signals, for outputting the high-frequency signals as frequency-modulated transmission millimeter-wave signals by periodically controlling bias voltage,
the voltage-controlled oscillating portion 21 being installed at an end of a first dielectric strip 37b (37a);
(b) a second dielectric strip 75 which is joined, along an arc having a radius of curvature r not less than the wavelength xcex, of the transmission millimeter-wave signals, to a straight portion 37b1 of the first dielectric strip 37b on the downstream side from the voltage-controlled oscillating portion 21 in the direction 71 for transmitting the transmission millimeter-wave signals of the first dielectric strip 37b (37a);
(c) a circulator 76 which has an input end 78, an input/output end 79 and an output end 80,
the circulator 76 being connected to the other end of the first dielectric strip 37b at the input end 78,
outputting transmission millimeter-wave signals inputted into the input end 78 to the input/output end 79, and
outputting reception signals inputted into the input/output end 79 to the output end 80;
(d) a third dielectric strip 77, one end of which is connected to the input/output end 79 of the circulator 76, and on the other end side of which is disposed a transmission/reception antenna 121;
(e) a terminator 112 which is connected to the output end 80 of the circulator 76;
(f) a fourth dielectric strip 114 having an end at which a reception antenna 122 is provided, for guiding received millimeter-wave signals;
(g) a mixer 82 for connecting the second dielectric strip 75 and the fourth dielectric strip 114 to mix respective signals transmitted to the second and fourth dielectric strips 75, 114 to generate intermediate frequency signals; and
(h) a pair of conductor plates 84, 85 which are placed in parallel at a spacing equal to or less than one half of the wavelength xcex of the millimeter-wave signals, in which spacing are disposed the first to fourth dielectric strips 37a, 37b; 75, 77, 114, the voltage-controlled oscillating portion 21, the circulator 76 and the mixer 82.
A millimeter-wave transmitting/receiving apparatus of the invention comprises:
(a) a high-frequency diode 33 which outputs high-frequency signals of millimeter-wave band;
(b) a first dielectric strip 37b (37a), one end of which is connected to the high-frequency diode 33, for propagating high-frequency signals outputted from the high-frequency diode 33;
(c) a pulse-modulating diode interposed between the first dielectric strip 37b (37a) or installed therealong, so that a bias voltage applying direction 72 coincides with an electric field direction of the high-frequency signals, for outputting transmission millimeter-wave signals which are pulse-modulated signals of the high-frequency signals by on-off of bias voltage;
(d) a second dielectric strip 75 which is joined, along an arc having a radius of curvature r not less than the wavelength xcex of the transmission millimeter-wave signals, to a straight portion 37b1 of the first dielectric strip 37b on the downstream side from the high-frequency diode of the first dielectric strip 37b (37a) in the transmission direction 71 of the transmission millimeter-wave signals;
(e) a circulator 76 which has an input end 78, an input/output end 79 and an output end 80,
the circulator 76 being connected to the other end of the first dielectric strip 37b at the input end 78,
for outputting transmission millimeter-wave signals inputted into the input end 78 to the input/output end 79, and
outputting reception signals inputted into the input/output end 79 to the output end 80;
(f) a third dielectric strip 77, one end of which is connected to the input/output end 79 of the circulator 76, and on the other end side of which is disposed a transmission antenna 121;
(g) a terminator 112 which is connected to the output end 80 of the circulator 76;
(h) a fourth dielectric strip 114 having an end at which a reception antenna 122 is provided, for guiding received millimeter-wave signals;
(i) a mixer 82 for connecting the second dielectric strip 75 and the fourth dielectric strip 114 to mix respective signals transmitted to the second and fourth dielectric strips 75, 114 to generate intermediate frequency signals; and
(j) a pair of conductor plates 84, 85 which are placed in parallel at a spacing equal to or less than one half of the wavelength xcex of the millimeter-wave signals, in which spacing between the conductor plates 84, 85 are disposed the first to fourth dielectric strips 37a, 37b; 75, 77, 114, the pulse-modulating diode, the circulator 76 and the mixer 82.
In the millimeter-wave transmitting/receiving apparatus of the invention it is preferable that the portion 37b1 of the first dielectric strip 37b on the downstream side is curved so as to make an arc having the radius of curvature r and the second dielectric strip 75 is linearly connected to the arc-shaped portion.
As shown in FIG. 13, in the millimeter-wave transmitting/receiving apparatus of the invention it is preferable that the mixer 82 has a construction of electromagnetically coupling an arc-shaped portion 115 midway in a transmitting direction 86 of the second dielectric strip 75 to a straight or arc-shaped portion 116 midway in a transmitting direction 88 of the fourth dielectric strip 114, so as to be in close proximity to each other.
As shown in FIG. 14, in the millimeter-wave transmitting/receiving apparatus of the invention it is preferable that the mixer 82 has a construction of joining, to a straight portion 118 of the fourth dielectric strip 114, the second dielectric strip 75 along an arc-shaped portion 119 having the radius of curvature r.
In the millimeter-wave transmitting/receiving apparatus of the invention it is preferable that the mixer 82 has a construction in which the second dielectric strip 75 is connected to the arc-shaped portion 118 of the fourth dielectric strip 114, having the radius of curvature r, so as to make a straight portion 119.
According to the invention, high-frequency signals of millimeter-wave band outputted by the high-frequency diode 33 are passed through the first dielectric strip 37a, and transmission millimeter-wave signals which are obtained by modulating the bias voltage of the variable capacitance diode 30 by a modulated wave which is periodically varied by a triangular wave or the like, are supplied through the first dielectric strip 37b to the input end of the circulator 76. The transmission millimeter-wave signals outputted from the input/output end 79 of the circulator 76 are radiated, through the third dielectric strip 77, from a transmission antenna 121 toward a target 104.
It is possible that high-frequency signals of millimeter-wave band are pulse-modulated to be converted into transmission millimeter-wave signals. In this case, a pulse-modulating diode such as a pin diode or schottky-barrier diode is interposed midway in a transmitting direction 71 of the first dielectric strips 37a, 37b, or installed therealong, so that a bias voltage applying direction coincides with an electric field direction of the high-frequency signals, for converting the high-frequency signals into pulses by on-off of bias voltage.
Reflection waves by the target 104 are received by a reception antenna 122 and supplied through a fourth dielectric strip 114 to the mixer 82. To the mixer 82, transmission millimeter-wave signals from the second dielectric strip 75 joined along an arc to the straight portion 37b1 of the first dielectric strip 37b are supplied. Thus, with the mixer 82, intermediate frequency signals mixed the reflection waves received from the reception antenna 122 and the transmission millimeter-wave signals from the second dielectric strip 75 can be obtained.
The reflection waves by the target 104 are also supplied to the transmission antenna 121, and supplied from the circulator 76 via the output end 80 of the circulator 76 to a terminator 112. The signals supplied to the terminator 112 are heat-consumed without generating reflection waves.
The mixer 82 may be constructed as shown in FIG. 13, or may be constructed as shown in FIG. 14.